1. Field of the Invention
The present invention relates to a hydraulic circuit which controls a piston movement of an injection cylinder which moves a plunger of a die-casting apparatus forward/backward.
2. Description of the Related Art
In general, in a die-casting apparatus, when an injection speed, an injection pressure, or the like, of a molten metal is inappropriate, various defects occur in a molded product. For example, when the injection speed is low or when the injection pressure is low, filling of a mold cavity with the molten metal becomes poor, and thus a defect occurs in a molded product. On the other hand, when the injection speed is high or when the injection pressure is high, filling of the mold cavity with the molten metal becomes good, but the molten metal enters between mating faces of the mold and burrs occur in a molded product.
Therefore, conventionally, for controlling an injection speed, an injection pressure, or the like, of a molten metal, a hydraulic circuit A1 or A2 as, shown in FIG. 9 is provided at an injection cylinder C which moves a plunger forward/backward, whereby the injection speed of the molten metal is controlled.
The hydraulic circuit A1 or A2 includes: an inflow circuit 3 which flows a pressure oil from a pressure oil source 1 such as a hydraulic pump or an accumulator through a switching valve 2 into a piston rear chamber R1 of the injection cylinder C; and an outflow circuit 5 which returns the pressure oil flowing out from a piston front chamber R2 of the injection cylinder C, to an on tank 4 through the switching valve 2. In the hydraulic circuit A1 for so-called “IN restriction” shown in FIG. 9(a), a flow control valve 6 is provided between the piston rear chamber R1 and the switching valve 2 in the inflow circuit 3. In such a hydraulic circuit A1 for IN restriction, in which the flow control valve 6 is provided, the pressure oil returned from the piston front chamber R2 through the outflow circuit 5 to the oil tank 4 by a movement of a piston P does not have resistance, and thus mechanically movable parts such as the piston P and a piston rod Pr connected thereto have great inertial forces, whereby a molten metal can be pressed into a cavity at a maximum pressure. Therefore, a mold for IN restriction, which has a small supply port, is installed in a die-casting apparatus which employs the hydraulic circuit A1 for IN restriction as an injection speed control circuit for the molten metal.
On the other hand, in the hydraulic circuit A2 for so-called “OUT restriction” shown in FIG. 9(b), a flow control valve 7 is provided between the piston front chamber R2 and the switching valve 2 in the outflow circuit 5. In such a hydraulic circuit A2 for OUT restriction, in which the flow control valve 7 is provided, the inertial forces of the mechanically movable parts such as the piston P and the piston rod Pr can be controlled by controlling a flow of the pressure oil flowing out from the piston front chamber R2, and thus it is easy to adjust the injection speed of the molten metal. However, the piston P receives resistance due to a back pressure generated when the flow at the flow control valve 7 is reduced, and the pressure which presses the molten metal into the cavity may be reduced. Therefore, a mold for OUT restriction, which has a large supply port, is installed in a die-casting apparatus which employs the hydraulic circuit A2 for OUT restriction as an injection speed control circuit for the molten metal.
As described above, regarding a hydraulic circuit of an injection cylinder which controls an injection speed of a molten metal, characteristics of the hydraulic circuit A1 for IN restriction and characteristics of the hydraulic circuit A2 for OUT restriction are greatly different from each other, and each hydraulic circuit needs a different mold according to its characteristics. Thus, for example, when a mold for OUT restriction, which has a large supply port, is installed in a die-casting apparatus which includes the hydraulic circuit A1 for IN restriction as a hydraulic circuit of an injection cylinder, the molten metal is rapidly supplied into a cavity of the mold while having a high pressure, and thus burrs occurs.
In contrast, as a hydraulic circuit of an injection cylinder, which is suitable for both a mold for IN restriction and a mold for OUT restriction, there is a circuit, as shown in FIG. 10, in which a first flow control valve 8 is provided in an inflow circuit 3 to a piston rear chamber R1, a second flow control valve 9 is provided in an outflow circuit 5 from a piston front chamber R2, and an opening degree of the second flow control valve 9 is controlled so as to correspond to an opening degree of the first flow control valve 8 (e.g., Patent Document 1: Japanese Laid-Open Patent Publication No. 60-33863).
According to this hydraulic circuit, when a mold for IN restriction is installed, control is performed such that the opening degree of the second flow control valve 9 is higher than the opening degree of the first flow control valve 8. On the other hand, when a mold for OUT restriction is installed, control is performed such that the opening degree of the second flow control valve 9 is reduced so as to be lower than the opening degree of the first flow control valve 8.